There's no problem with adding fields and views, but don't change anything that is pre-existing. Do all your work in a template, and be very careful with the management of that template. There are two dangers:
(a) losing your modified template due to inadvertant replication from a newly installed server that has a standard NAMES.NTF that is newer than your custome version, and
(b) having two different templates for the NAB on different servers such that $DESIGN on each server re-inherits conflicting designs every night causing a "Design Storm" that can bring your whole network down.
Do not name it NAMES.NTF, or it may be overwritten by a software upgrade. Make sure that the template name is not the default (StdNotesAddressBoook). You can either make it a replica of the original NAMES.NTF, or a copy.
If it is a replica, be sure to remove the original, be sure to let it replicate to all servers, and be especially sure that any time a new Notes version is loaded that you merge any changes in the new version's NAMES.NTF into your template and then delete that NAMES.NTF from your server before you allow the upgraded server to replicate with any other server.
If it is a copy instead of a replica, it is best that you uncheck the setting that allows replication of the template name for your NAB and make sure that only one server in your organization is set up to inherit the NAB design from your modified template.
Just for insurance it might be a good idea to use the ACL or selective replication to insure that your main hub server never accepts NAB design changes from any other server.
For Notes R4 and R5, the preferred way of adding fields to the NAB is to use subforms.